First sea clock

The H4, built by John Harrison (1693-1776), was the first timepiece ever constructed that was accurate enough to allow the calculation of longitude during navigation by seafarers. In tests at sea, this large revolutionary pocket watch lost just five seconds in six weeks. This was three times the accuracy required to win the 'Longitude Prize'. The H4 was first tested at sea when John Harrison's son, William, set sail with it for Jamaica on 18 November 1761. The Longitude Prize, a £20,000 award, was offered by the Board of Longitude in 1714, set up by the British Parliament, to anyone who could solve the problem of accurately calculating longitude at sea.